This patent specification is directed to a method for determining a registration error of a feature on a mask.
This patent specification is also directed to a method for simulating an aerial image from pattern specifications of a mask.
The patent specification is also directed to a position measuring device for carrying out the method.
In lithography for semiconductor component fabrication, the patterns of masks, which are also referred to synonymously as reticles, are projected by means of scanners or steppers onto wafers coated with a light-sensitive layer, the resist. Masks can be configured, for example, as “binary masks,” with chromium patterns on quartz glass, or as phase-shift masks. For EUV lithography applications, reflective masks are used. Templates for the nanoimprint process are another variety of mask. With the aid of mask inspection microscopes or position measuring devices, the pattern of a reticle is projected by means of lenses onto a light-sensitive local-resolution detector, such as, for example, a CCD (Charge Coupled Device) chip.
By means of a position measuring device (registration tool), features on a mask that are specifically designated as registration patterns or as markers, for example squares, crosses or angles of specific shapes, are measured and compared with their nominal positions. Positions on the mask of features that are details of the patterns used on the mask are also measured. This is known as “real pattern registration.” The deviation of the nominal position of a feature from its actual position on the mask is the placement error, which is also known as the “registration” or “registration error.”
The measurement of masks makes it possible to check the positional accuracy of the patterns on the mask during the electron-beam mask-writing process. In addition, by measuring the patterns of an existing set of masks, the deviations of the pattern positions on the various masks can be adjusted to each other for the individual lithographic layers.
To check the positions of features, an aerial image of a detail of a mask is captured with a position measuring device. The mask rests for this purpose on a stage (also known as an object table or traversing unit) that serves to move the mask in the direction of the mask plane, in order to position a desired detail in the image field of the positioning measuring device so that the image can be captured with a detector. Before the measurement is performed, the mask is oriented on the stage in such a way that its position on the stage is known. Alternatively, relative alignment with special alignment features can be performed. The position determination is then made in relation to these features, which are also known as alignment markers. The image can thus be assigned unequivocally to the absolute or relative position of the detail on the mask. By determining the position of the pattern in the captured image, it becomes possible to compare the nominal and actual positions of the patterns on the mask, and thus to calculate the placement error.
Metrological requirements for placement error determination currently stand at 1 nm. Feature placement errors can be determined, for example, via methods for determining the positions of these features in aerial images. Examples of the known methods are threshold value methods and correlation methods. Accuracy requirements for the to-be-determined position or placement error of a feature are higher than the resolution of an aerial image captured by the position measuring device. In current devices, the edge lengths of a pixel of an aerial image correspond, for example, to 25 nm in mask dimensions. Methods have been developed that permit so-called subpixel-accurate position determination of a feature in an aerial image.
For example, DE1 2006059431 describes a method for determining the position of a pattern on a substrate with subpixel accuracy in relation to a reference point on the substrate by overlaying the pattern with a reference pattern.
Known from DE10047211 is a method for determining the positions of edges of a pattern with subpixel accuracy. Here, the measured edge profiles are compared with model intensity profiles to find the position of best fit.
DE10337767 discloses a method for determining the distance between a reference element and a pattern for measuring the overlay shift.
DE102007033815 discloses a method for determining the distance between a reference element and a pattern by means of two pattern elements for measuring the overlay shift, wherein in the reference element or in the pattern, one of the pattern elements whose position is to be determined is amplified.
These methods have not yet made it possible to achieve the requirements imposed on the next generation of position measuring devices.